1. Field of the Invention
The present invention relates to the design and manufacture of and includes monolithically integrated diodes for use in various applications including in planar, thin-film, photovoltaic devices such as solar cells.
2. Description of the Prior Art
Photovoltaic (PV) cells generate an electric current when exposed to light (a photocurrent). PV cells thus have a wide variety of potential uses, including functioning as a power supply in certain terrestrial and space applications, acting as a photosensor to detect either a binary on-off presence of light as is required for certain security systems, and acting as a photosensor to detect varying intensities and/or wavelengths of light as is required for a variety of photographic and videographic applications. PV cells may produce photocurrent in response to a wide range or specific narrow band of the electromagnetic spectrum, including that band defined by the visible spectrum. In some applications, a PV cell that is transparent or translucent to a first frequency of light but produces a photocurrent in response to a second frequency is placed on top of a PV cell that produces a photocurrent in response to a first frequency. Although a PV cell produces current when exposed to light, when shadowed or shaded, it behaves as a diode. When a PV cell is described herein as being in a shadowed or shaded state, it means that it is not receiving sufficient light of the relevant wavelength or wavelengths to produce photocurrent.
A common practice in configuring PV cells, particularly when used in a power supply, is to place multiple cells in series. When a portion of the series or string of cells is shadowed, the shadowed cell (or cells) acts as a diode in reverse bias to the remainder of the series or string. As a result, the cell (or cells) acting as diodes in reverse bias tends to gain heat, which consequently damages that cell and potentially damages nearby cells. This heating is evidence of what is termed xe2x80x9cdestructive breakdown,xe2x80x9d which is detrimental to a cell. Moreover, the energy consumed in heating and breaking down the cell is wasted, thus producing a less efficient device.
One method that has been applied to overcoming the problem of shadowed cells is to incorporate diodes into the design of the series or string. These diodes traditionally have been discrete components that have been attached to the PV cell by soldering or similar techniques. The addition of these discrete diodes provides an alternate electrical path in the event of a shadowed or shaded cell.
Unfortunately, the additional diodes add to a module""s weight, thickness, complexity, and cost of manufacture, while decreasing its reliability. Moreover, many of the connection techniques used to attach discrete diodes impose additional constraints (e.g., rigidity) on the cell, further limiting its usefulness. Examples of designs that utilize discrete diodes include U.S. Pat. Nos. 6,255,793, 6,103,970, and 4,577,051. Other techniques which have been used include diodes which are integrated into the design of the cell. These may rely on xe2x80x9cCxe2x80x9d or xe2x80x9cSxe2x80x9d shaped interconnects to connect adjacent cells. An example of such a design is U.S. Pat. No. 5,616,185. These designs reduce the active area of the cell, thereby reducing efficiency.
Other designs involving integrated diodes make use of both sides of the PV cell. Some examples of this type of design include U.S. Pat. Nos. 5,580,395 and 4,323,719. Because these designs include additional layers on the bottom or reverse of the cell, they make the cell inherently thicker, and consequently heavier. Additionally, these designs increase the complexity of the design, and the cost of both materials and manufacture. Other techniques for creating integrated diodes do so by means of special doping techniques. Examples of this kind of design include U.S. Pat. Nos. 5,990,415, 5,389,158, 5,389,158, 5,248,346, and 4,933,022.
Another technique for creating integrated diodes includes adding additional partial layers to the surface of the cell and connecting these xe2x80x9cintegrated diodesxe2x80x9d to the cell using integrated circuit techniques. An example of this type of design is U.S. Pat. No. 4,759,803.
An ideal design would provide an individual bypass diode for each PV cell, however, cost, size, and weight constraints may limit the designer to include only one bypass diode for every two or more series PV cells. In these configurations, the total number of cells which are in parallel to each bypass diode may be bypassed if only one of the PV cells of the group is shadowed, yielding less than optimal efficiency. Moreover, the larger bypassed voltage may require more robust diodes to avoid exposing the bypass diode to a voltage greater than or equal to its breakdown voltage.
The present invention relates to the design and manufacture of and includes monolithically integrated diodes for use in various applications including planar, thin-film PV devices such as solar cells. The design of the present invention is exemplified by an embodiment based upon a string of PV cells into which diodes have been monolithically integrated. The design produces a series or string of PV cells that have reduced weight, thickness, cost, and complexity while achieving increased reliability compared to the prior art. Other advantages of the present invention include its ability to be used in flexible thin film devices, its ability to extend the life span of PV cell series or strings, and its ability to increase manufacturing output. The integrated diode of the present invention is also capable of acting as a bypass diode, or as a blocking diode, to prevent the reverse flow of current from, for example, the electrical bus or parallel series or string to which the series or string may be connected.
One embodiment of the present invention, for example, overcomes the problem of discrete components through a monolithic integration technique which permits diodes to be fully integrated without the need for discrete components.
One embodiment of the present invention is created by providing a photovoltaic cell. This cell may comprise a substrate, and deposited on this substrate there may be a conducting layer, upon which a p-type absorber layer may be deposited. The cell may further comprise an n-type window layer deposited on this p-type absorber layer. The substrate may, for example, comprise an electrically insulating top surface, such as Upilex(copyright), polyimide, polyphenylene benzobis oxazole (PBO), polyamide, polyether ether ketone, or metallic foils coated with one of these electrically insulating materials. The conducting layer may, for example, comprise molybdenum, transparent conductive oxides, brass, titanium, nickel, or nickel-vanadium. The p-type absorber layer may, for example, comprise a device made of copper-indium-gallium-selenide, copper-indium-selenide, copper-aluminum-selenide, copper-indium-aluminum-selenide, or other compounds like those mentioned that substitute (in whole or part) silver for copper or sulfur for selenide or both. The n-type window layer may, for example, comprise cadmium sulfide, cadmium zinc sulfide, zinc sulfide, zinc selenide, cadmium selenide, zinc-indium selenide, or indium selenide. Next, one may remove a portion of the cell including the n-type window layer, the p-type absorber layer, and the conducting layer, thereby producing a trench or groove defined by the edges of the remaining portions of the cell. A preferred way to remove these layers is by means of laser scribing. Other techniques to remove these layers include, for example, chemical etching, electronic etching (such as, for example, e-beam writing or electron scribing), and mechanical scribing. Chemical etching may require masking to prevent unwanted removal of other portions of the layers. These techniques are sometimes referred to as scribing, and the resulting trench or groove as a scribe.
Depending on the method of removal used, it may be desirable to clean the groove or trench to remove debris or other by-products of the removal process. One may also apply an insulating material to fill this trench. This insulating material may, for example, comprise a resistive ink, a dielectric, a pottant, or an encapsulant. Several methods for applying insulating material such as resistive ink include bubblejet deposition, inkjet deposition, or other types of liquid media printing techniques including screen printing.
Additionally, one may remove a portion of the cell parallel to the insulating material by, for example, removing the n-type window layer and p-type absorber layer, thereby defining a second trench. This removal of a portion of the cell may be performed by, for example, the above described techniques.
Following this removal, a layer of translucent conductive oxide, such as, for example, indium tin oxide (ITO), may be applied to the surface of the cell and may include the insulating material and trench. One may again remove a portion of the cell that may include the translucent conductive oxide layer, the n-type window layer, and the p-type absorber layer, forming a trench defined by the edges of the remaining cell. This removal of a portion of the cell may be performed by, for example, the above described techniques.
An opaque layer may also be added to a portion of the cell corresponding to the desired diodes. The opaque layer may comprise any opaque material, such as, for example, a metal, in particular, aluminum, molybdenum, silver, or titanium may be desirable. Other translucent materials that are sufficiently opaque may be used. Other materials that are opaque at certain times, such as, for example materials such as are used in liquid crystal displays may be used, and may provide greater flexibility in the design of circuits that integrate embodiments of the present invention. Other materials may be used in the opaque layer such as, for example, materials that are polarized in such a manner as to sufficiently shade or shadow the underlying layers. Several ways to apply the opaque layer include physical vapor deposition, chemical vapor deposition, sputtering, evaporation, and chemical bath deposition.
In a further embodiment of the present invention, a module of photovoltaic cells may be manufactured using the method of the present invention. One may begin, for example, by providing a substrate. Preferably, this substrate may be a flexible polyimide substrate, such as, for example, Upilex(copyright), and may preferably be flat, planar, and rectangular. A conductive layer may be deposited on this substrate and may, for example, comprise molybdenum. On the conductive layer, a p-type absorber layer may be deposited, and may comprise a copper-indium-gallium-selenide device. On the p-type absorber layer, an n-type window layer may be deposited, and may, for example, comprise cadmium sulfide. The intermediate products of this process will be referred to as xe2x80x9cthe sheet.xe2x80x9d This term is not meant to describe any functionality or dimensions, or to provide any limitation, but merely to provide a shorthand name for the product of the processes up to the point at which reference is to be made.
Additionally, one may remove a first portion of the sheet, including a portion of the n-type window layer, the p-type absorber layer, and the conductive layer. The portions of these layers may be removed so as to form a first trench or groove. This pattern of this first trench or groove in the sheet may be adapted to provide for a plurality of PV cells. Each PV cell may be considered as a two terminal device. For each PV cell desired, a first trench or groove may be included. Furthermore, a layer of electrical insulator, such as, for example, resistive ink may be applied to the sheet. The electrical insulator may be applied so as to replace the portion of the sheet removed by the first trench or groove.
An additional step in an embodiment of the present invention may be to remove a second portion of the sheet, including a portion of the n-type window layer and the p-type absorber layer, to form a second trench or groove. The second trench or groove may preferably be formed parallel to the first trench or groove. Preferably, the second trench or groove is near the first trench or groove, and may, if desired, be partially overlapping the first trench or groove. The second trench or groove may be formed prior to or during the application of the electrical insulator layer, if desired.
A further step in an embodiment of the present invention may be to apply a translucent conductive layer to the sheet including the n-type window layer, the conductive layer, and the second trench or groove. One may also remove a portion of the sheet including the translucent conductive layer, the n-type window layer, and the p-type absorber layer, to form a third trench or groove.
A third trench or groove may preferably be formed parallel to the second trench or groove. Preferably, the third trench or groove is near the second trench or groove, and may, if desired, be partially overlapping the second trench or groove. The pattern in which the trenches or grooves are arranged will determine the size and location of the PV cells.
Finally, an opaque layer may be applied to those portions which are intended for use as diodes. The opaque layer may comprise any opaque material, such as, for example, a metal. Other translucent materials that are sufficiently opaque may be used. Other materials that are opaque at certain times, such as, for example materials such as are used in liquid crystal displays may be used, and may provide greater flexibility in the design of circuits that integrate the present invention. Other materials may be used in the opaque layer such as, for example, materials that are polarized in such a manner as to sufficiently shade or shadow the underlying layers.
In an embodiment of the present invention, it may be convenient to locate the diodes along the edges of the module. Other locations which are a priori known to be likely to be shadowed or shaded may also provide good locations for diodes. The area of a diode of the present invention will be determined by the characteristics of the materials used, particularly the reverse-bias characteristics of the p-type absorber layer. Preferably the diodes should exhibit low internal resistances in forward bias and should not be required to face voltages approaching their breakdown voltages.
Cells or diodes may be isolated one from another as desired, by means of, for example, trenches or grooves. Additional electrical and environmental protection may be accomplished by, for example, encapsulating one or more cells or diodes in a dielectric material. This material may, for example, also fill the trenches or grooves and provide the benefit of additional isolation.
An object of the present invention is to provide a method of manufacturing diodes integrated into PV devices such as solar cells. It is another object of the present invention to provide a method of manufacturing diodes as bypass diodes integrated into PV devices such as solar cells. Yet another object of the present invention is to provide a method of manufacturing diodes as blocking diodes integrated into PV devices such as solar cells.
Another object of the present invention is to provide a method of manufacturing thin-film PV devices that are flexible, yet which comprise diodes, thereby eliminating the requirements made by discrete diodes, including for example, limitations of rigidity imposed by the diodes themselves and by the associated soldered connections.
It is a further object of the present invention to provide a low-cost, low-weight protected array of PV cells. This array may be of particular use in space applications, but may also be of use in terrestrial power generation. Another object of the present invention is to provide a protected array of PV cells which is thin, thereby saving volume, which may be of particular benefit in space applications.
Another object of the present invention is to provide a method of manufacturing diodes integrated into PV cells that does not significantly increase the complexity of the manufacture of PV cells. Thus, it is an object of the present invention to provide a method of manufacturing diodes integrated into PV cells that is less process intensive, and which may be accomplished using a relatively simple stacking sequence.
It is a further object of the present invention to provide a method of manufacturing a protected array of PV cells, in which the cells may be as small as 0.3 cm by 31 cm, thus allowing the production of 100V in less than 80 cm. In such a configuration, discrete diodes would have to be laid practically end to end. Given the high voltage of such an array, protection by means of diodes may be important.
It is an object of the present invention to provide a method of manufacturing a protected array of PV cells which reduces the complexity of manufacture by eliminating the steps of pick-and-place, bonding, and soldering.